1. Field of the Invention
The present invention relates to an illumination system, and more particularly, to an illumination system for a projection apparatus.
2. Descriptions of the Related Art
Because common light emitting diodes (LEDs) feature high energy conversion efficiency, long service lives, high stability and capability of being instantaneously switched on/off, they have gradually replaced conventional ultra high pressure (UHP) mercury bulbs as light sources of projection apparatuses. When LEDs are adopted as the light source of a projection apparatus, the design and arrangement of internal optical components thereof need to be considered to achieve a desirable light utilization ratio and desirable light distribution uniformity. A common approach is to directly use red LEDs, blue LEDs and green LEDs as the light source of the projection apparatus. However, it is known that the green LEDs have low light emission efficiency and the red LEDs have a high requirement on heat dissipation. Consequently, if the red LEDs, the blue LEDs and the green LEDs are directly used as the light source of the projection apparatus without being appropriately adjusted beforehand, it will be difficult to adjust the overall color uniformity, which makes use of the projection apparatus very inconvenient.
In this case, the light source of the projection apparatus may be provided in another way, i.e., by using a blue LED or an ultraviolet (UV) LED in combination with a color wheel. However, if only the light emitted by a single blue LED or a single UV LED to the color wheel is used as the light source of the projection apparatus, then the luminance of an image projected will be significantly reduced. Moreover, LEDs are a kind of light source with a large divergence angle and a large etendue; as a result, the light source of the projection apparatus will suffer from an inadequate intensity and the light energy emitted from the LED will not be used efficiently if the etendue cannot be reduced for more efficient light concentration.
Accordingly, an urgent need exists in the art to solve the aforesaid problem by increasing the utilization ratio of the light source of the projection apparatus and enhancing the intensity of the whole light source.